The subject of the present invention is a pneumatic brake-booster with additional hydraulic boosting and a method for boosting braking using such a booster. An object of the invention is to make the operation of the additional boost effect more reliable and make a braking action perfectly linear as a function of a braking command. The field of application of the invention is more generally that of the general control of the braking of a vehicle.
A pneumatic brake-booster comprises, in principle, a variable-volume front chamber separated from a rear chamber, the volume of which is also variable, by a partition formed by a sealed and flexible diaphragm and by a rigid skirt plate. The rigid skirt drives a pneumatic piston bearing, via a push rod, on a primary piston of a master cylinder of a hydraulic braking circuit, typically a tandem master cylinder. The front chamber, on the master cylinder side, is connected pneumatically to a source of vacuum. The rear chamber, on the opposite side to the front chamber, and placed on the brake pedal side, is connected pneumatically, in a way controlled by a valve, to a source of driving fluid, typically air at atmospheric pressure. At rest, that is to say when a driver is not pressing on the brake pedal, the front and rear chambers are connected together, where as the rear chamber is isolated from atmospheric pressure. Under braking, the front chamber is first of all isolated from the rear chamber, then air is let into the rear chamber. This letting-in of air has the effect of driving the partition and of implementing pneumatic boosting of the braking.
Hydraulic brake boosting effects are known from elsewhere. Typically, an electric motor is connected to a hydraulic pump which injects a fluid under pressure into the braking circuits when these are called upon. Control of this electric motor is provided by measuring the pressures obtaining in the front and rear chambers of the pneumatic brake-booster. Use is therefore made of two pressure detectors, which are connected pneumatically to each of these chambers, so as to measure the pressure. These detectors provide electrical signals that represent these pressures.
The additional hydraulic boost system has the main objective, as is known, of contriving to prevent the wheels from locking under sudden braking. Such a system, which is known as an anti-lock braking system, or as ABS, allows the hydraulic pressure in the braking circuit to be modulated. This system makes it possible to apply, or alternatively, not to apply, additional hydraulic pressure to the hydraulic circuit so that the pressure applied exceeds or does not exceed a limiting pressure beyond which the wheels will lock up.
A change to these additional hydraulic boost braking circuits has been dictated by the reduction in weight of the vehicles. What has happened is that this reduction has led to a reduction in the size of the pneumatic brake-booster, the size of the front and rear chambers. Because of the size reduction, and because the high pressure used is atmospheric pressure (which is practically always the same), the boost effect afforded by the pneumatic boosting has seen a reduction in its effectiveness. The object of additional hydraulic boosting is therefore to afford additional boosting, which tends to apply to the hydraulic circuit a raised hydraulic pressure so that a hydraulic pressure which is higher than the one needed to cause the wheels of a vehicle being braked to lock up can be reached.
In practice, the pneumatic boost function links the force of effort exerted by a driver to the hydraulic pressure of the braking circuit, namely the effectiveness of the braking, in a linear way. However, this linear relationship is achieved only as long as the high pressure let into the rear chamber can exert a boosted braking effort. In practice, there is equilibrium in the position of a brake pedal. This equilibrium results, on the one hand from the effort exerted by the driver added to the pneumatic boosting, and, on the other hand, from the hydraulic reaction of the braking circuit. In order to avoid driver fatigue, a ratio between the pneumatic boosting and this driver effort, which is of the order of five, or other values depending on the various systems are allowed.
From the moment when the rear chamber of the pneumatic booster is subjected to atmospheric pressure (and can no longer be subjected to a higher pressure) and the front chamber is subjected to the maximum depression that the vacuum pump can produce, the pneumatic boosting no longer comes into effect. Under these conditions, the additional component of the braking effort is provided only by the driver. The pressure obtaining in the hydraulic circuit when this phenomenon occurs is known as the saturation pressure.
The curve of the correspondence between the pressure in the master cylinder and the force exerted by the driver therefore experiences an initial increase along a slope known as the boost slope, which is fairly steep as far as this saturation pressure value. It then evolves with a far shallower slope, due only to the effort on the part of the driver.
When the pneumatic boost braking system was bulky, this saturation pressure was above the pressure at which the wheels of the vehicle locked up. It was then left to the driver, or alternatively to an anti-lock braking system, to keep this other problem in check. However, because of the reduction in the size of pneumatic boosters, the saturation pressure is now reached in the hydraulic circuit before the pressure at which the wheels lock up is reached. To this end, an additional hydraulic boost circuit, typically a hydraulic pump, takes over from this pneumatic boosting action.
This additional boosting, for good driveability, has nontheless to occur in continuity with the efforts deployed by this driver. This means that the ratio between the effort exerted by the driver and the hydraulic pressure acting on the wheels have to be the same as, or similar to, the ratio that already existed at the time that the additional hydraulic boosting comes into operation. Corresponding to the saturation pressure in the hydraulic circuit, there is an effort known as the saturation effort for which this saturation pressure is reached. The effort exerted by the driver is therefore measured in the additional hydraulic boost circuits, the saturation effort is subtracted, and the difference is multiplied by the coefficient of amplification that already existed during pneumatic boosting.
By taking this approach the result obtained as far as the driver is concerned is therefore that the boosting occurs always with the same effectiveness, whether it is pneumatic or hydraulic in origin. The driver is unaware of the difference. In order to measure the saturation pressure or the saturation effort, there are various conceivable systems, the principle of which is to compare with one another the pressures that exist at the time of this saturation in the front chambers, rear chambers or various points of the hydraulic circuit.
However, under certain circumstances such an additional hydraulic boost system operates in an abnormal and troublesome way. This abnormal and troublesome operation stems from a sharp increase in the vacuum in the front chamber during braking. This sharp increase in the vacuum may, for example, be brought about by engaging a lower gear in the gearbox. This lower gear itself leads to the engine turning over faster (engine braking) which leads to a greater intake and therefore a stronger depression created in the front chamber.
The origin of this greater depression in the front chamber may also be the result of operation of a non-return valve present on the intake of the vacuum into this front chamber. What happens is that a calibrated leakage of low value, for example 15 millibar per second, causes an increase in pressure in the front chamber. It is possible for the pressure in the front chamber, as a result of this leakage, to exceed the set pressure of this non-return valve. Under these conditions, the front chamber is once again subjected to the vacuum. The set pressure of a non-return valve such as this is of the order of 25 millibar. As a result, a sudden depression of the order of 25 millibar may be applied to the front chamber.
If both phenomena, the engaging of a lower gear in the gearbox, and the refreshing of the vacuum in the front chamber, occur simultaneously, it is possible for the front chamber to experience an additional depression, of the order of 50 millibar. In this case, and if the pneumatic booster is in the saturation region, misadjustment of the additional hydraulic boost system occurs. Everything therefore happens as if this sharply applied depression were interpreted by the additional hydraulic boost circuit as a sudden effort applied by the driver. The amplification phenomenon afforded by the additional hydraulic boosting then leads to excessive braking which tends to cause the wheels to lock up immediately. In practice, this phenomenon is compensated for by the ABS cutting in, so that no decremental effect on driving occurs. However, in the exceptional case where such a situation might occur, a solution such as this is disagreeable and unsatisfactory.